[ATECHPCS]

Problem

A measure is amount units of name weighing value grams in total, and the app renders a serving as <amount> <name>. update_custom_food only writes name and value, so it cannot fix a food whose default measure has a quantity other than 1.

Example from a real account: a custom food whose default measure had quantity 400 and type "Weight". After update_custom_food(food_id, serving_name="1 serving", serving_grams=250) the stored measure was

{"id": 310790759, "name": "1 serving", "amount": 400, "value": 250, "type": "Weight"}

get_food_details reads that back as 1 serving = 250 g, but the iOS editor showed the serving as "400 1 serving" with no serving weight.

Change

• serving_grams now also resets the measure's quantity to 1 and its type to "Atomic", the same shape create_custom_food writes.
• New optional serving_quantity sets the quantity explicitly (alone, or together with serving_grams).
• A plain number leading serving_name ("1 serving", "2 cookies") is moved into the quantity, unless serving_quantity is given. The server already does this for a new measure: add_custom_food(serving_name="10 pieces") sends amount: 1.0 and reads back as amount: 10, name: "pieces". It does not do it when an existing measure is re-sent, so an update with "1 serving" would render as "1 1 serving".
• Any other serving_name alone still only renames.
• serving_grams and serving_quantity must be positive finite numbers.
• get_food_details reports each measure's quantity and type, so the result can be checked.

Verification

• Against the live API: re-sending the food through /api/v2/add_food with amount: 1, type: "Atomic" on the existing measure id returned SUCCESS, and get_food read back the same measure id with the new quantity, type, name and weight. The other measures and the per-100g nutrients were unchanged.
• One existing assertion changes: serving_name="1 cup" now stores name cup with quantity 1.
• New tests in tests/test_custom_food_edit.py; ruff check and the full suite pass (162 tests).